It is well known that a mode of an electromagnetic ("EM") field may be described as the sum of two quadrature components whose time variations are given by sine and cosine functions, respectively. The amplitude of each quadrature component is a variable having a complementary and uncertainty relationship with the other. The variances in these components and the resulting variations in the EM field in a vacuum state having long been recognized to generate a "shot noise" level providing a limit on the precision of measurements made with an EM field.
Quantum mechanics has appreciated that the noise level can be reduced below the shot level by making the variances unequal; reduced fluctuations in one quadrature may be achieved by increasing the fluctuations in another quadrature (the so-called "squeezed states"). Such reductions would make possible significant advances in the precision of any EM field dependent measurement or process, such as spectroscopy, interferometry, communications and information storage.
Measurement techniques which avoid inducing fluctuations in the characteristic to be measured are called quantum nondemolition ("QND") measurements because such techniques ideally do not perturb or destroy the quantum property being examined. The nature of QND measurement has been discussed in such publications as Braginsky, et al., Quantum Nondemolition Measurements, Science, pp. 547-557, Vol. 209, No. 4456 (1 Aug. 1980); Hillery, et al., Quantum noise and quantum nondemolition measurements, Physical Review D, pp. 3137-3158, Vol. 25 No. 12 (15 June 1982); and, Milburn, et al., Quantum nondemolition measurements on coupled harmonic oscillators, Physical Review A, pp. 2804-2816, Vol. 27, No 6 (June 1983).
One successful QND measurement technique involves monitoring one quadrature component of an EM field while noise arising as a result of such monitoring is added entirely into a second quadrature component of the EM field. Because this measurement process evades the injection of noise back into one quadrature component of the EM field, in it is commonly referred to as "back-action evasion".
Several back-action evading ("BAE") schemes for achieving QND measurement in the optical region of the EM spectrum are illustrated in the article by B. Yurke entitled "Optical back-action-evading amplifiers", J. Opt. Soc. Am. B, pp. 732-738, Vol. 2, No. 5 (May 1985), which article is hereby incorporated by reference. One BAE scheme presented in Yurke involves pumping a nonlinear medium at the sum and difference of two different frequencies (which are referred to by Yurke as the signal frequency and readout frequency). As explained in Section 4 of the Yurke article, optical BAE may be achieved by the combination of frequency converting and parametrically amplifying both the input and output light beams. However, the complexity of frequency converters and their potential for introduction of additional sources of noise make this approach difficult to implement as a practical QND measurement device.
We have found a simple scheme for QND measurements in which BAE is accomplished by the conversion of polarizations of the EM field with parametric amplification.